3,232 research outputs found
Resonance superfluidity in a quantum degenerate Fermi gas
We consider the superfluid phase transition that arises when a Feshbach
resonance pairing occurs in a dilute Fermi gas. We apply our theory to consider
a specific resonance in potassium-40, and find that for achievable experimental
conditions, the transition to a superfluid phase is possible at the high
critical temperature of about 0.5 T_F. Observation of superfluidity in this
regime would provide the opportunity to experimentally study the crossover from
the superfluid phase of weakly-coupled fermions to the Bose-Einstein
condensation of strongly-bound composite bosons.Comment: 4 pages, 3 figure
Electro-oxidative depolymerisation of technical lignin in water using platinum, nickel oxide hydroxide and graphite electrodes
In order to improve the lignin exploitation to added-value bioproducts, a mild chemical conversion route based on electrochemistry was investigated. For the first time, soda lignin Protobind™ 1000 (technical lignin from the pulp & paper industry) was studied by cyclic voltammetry to preliminarily investigate the effect of the main reaction parameters, such as the type of electrode material (platinum, nickel oxide hydroxide, graphite), the pH (12, 13, 14), the scan rate (10, 50, 100, 250 mV s-1), the substrate concentration (2, 20 g L-1) and the oxidation/reduction potential (from -0.8 to +0.8 V). Under the optimal reaction conditions among those tested (NiOOH electrode, pH 14, lignin 20 g L-1, 0.4 V), the electro-oxidative depolymerisation of lignin by electrolysis was performed in a divided cell. The reaction products were identified and quantified by ultra-pressure liquid chromatography coupled with mass spectrometry. The main products were sinapic acid, vanillin, vanillic acid, and acetovanillone. The obtained preliminary results demonstrated the potential feasibility of this innovative electrochemical route for lignin valorisation for the production of bio-aromatic chemicals
Feynman path-integral treatment of the BEC-impurity polaron
The description of an impurity atom in a Bose-Einstein condensate can be cast
in the form of Frohlich's polaron Hamiltonian, where the Bogoliubov excitations
play the role of the phonons. An expression for the corresponding polaronic
coupling strength is derived, relating the coupling strength to the scattering
lengths, the trap size and the number of Bose condensed atoms. This allows to
identify several approaches to reach the strong-coupling limit for the quantum
gas polarons, whereas this limit was hitherto experimentally inaccessible in
solids. We apply Feynman's path-integral method to calculate for all coupling
strengths the polaronic shift in the free energy and the increase in the
effective mass. The effect of temperature on these quantities is included in
the description. We find similarities to the acoustic polaron results and
indications of a transition between free polarons and self-trapped polarons.
The prospects, based on the current theory, of investigating the polaron
physics with ultracold gases are discussed for lithium atoms in a sodium
condensate.Comment: 13 pages, 3 figure
Collective ferromagnetism in two-component Fermi-degenerate gas trapped in finite potential
Spin asymmetry of the ground states is studied for the trapped
spin-degenerate (two-component) gases of the fermionic atoms with the repulsive
interaction between different components, and, for large particle number, the
asymmetric (collective ferromagnetic) states are shown to be stable because it
can be energetically favorable to increase the fermi energy of one component
rather than the increase of the interaction energy between up-down components.
We formulate the Thomas-Fermi equations and show the algebraic methods to solve
them. From the Thomas-Fermi solutions, we find three kinds of ground states in
finite system: 1) paramagnetic (spin-symmetric), 2) ferromagnetic (equilibrium)
and 3) ferromagnetic (nonequilibrium) states. We show the density profiles and
the critical atom numbers for these states obtained analytically, and, in
ferromagnetic states, the spin-asymmetries are shown to occur in the central
regions of the trapped gas, and grows up with increasing particle number. Based
on the obtained results, we discuss the experimental conditions and current
difficulties to realize the ferromagnetic states of the trapped atom gas, which
should be overcome.Comment: submit to PR
Quasiparticle spectrum and dynamical stability of an atomic Bose-Einstein condensate coupled to a degenerate Fermi gas
The quasiparticle excitations and dynamical stability of an atomic
Bose-Einstein condensate coupled to a quantum degenerate Fermi gas of atoms at
zero temperature is studied. The Fermi gas is assumed to be either in the
normal state or to have undergone a phase transition to a superfluid state by
forming Cooper pairs. The quasiparticle excitations of the Bose-Einstein
condensate exhibit a dynamical instability due to a resonant exchange of energy
and momentum with quasiparticle excitations of the Fermi gas. The stability
regime for the bosons depends on whether the Fermi gas is in the normal state
or in the superfluid state. We show that the energy gap in the quasiparticle
spectrum for the superfluid state stabilizes the low energy energy excitations
of the condensate. In the stable regime, we calculate the boson quasiparticle
spectrum, which is modified by the fluctuations in the density of the Fermi
gas.Comment: 12 pages, 3 figure
Enhancement and suppression of spontaneous emission and light scattering by quantum degeneracy
Quantum degeneracy modifies light scattering and spontaneous emission. For
fermions, Pauli blocking leads to a suppression of both processes. In contrast,
in a weakly interacting Bose-Einstein condensate, we find spontaneous emission
to be enhanced, while light scattering is suppressed. This difference is
attributed to many-body effects and quantum interference in a Bose-Einstein
condensate.Comment: 4 pages 1 figur
Isospin Breaking in the Pion-Nucleon Coupling from QCD Sum Rules
We use QCD sum rules for the three point function of a pseudoscalar and two
nucleonic currents in order to estimate the charge dependence of the pion
nucleon coupling constant coming from isospin violation in the
strong interaction. The effect can be attributed primarily to the difference of
the quark condensates . For the splitting
we obtain an interval of to , the uncertainties coming mainly from the input
parameters. The charged pion nucleon coupling is found to be the average of
and . Electromagnetic effects are not included.Comment: 18 pages (REVTeX) + 2 figures (as PostScript), to be published in
PRC, replaced with final version: inclusion of pi-eta mixing and N -> N*
transition
Microscopic Dynamics in a Strongly Interacting Bose-Einstein Condensate
An initially stable 85Rb Bose-Einstein condensate (BEC) was subjected to a
carefully controlled magnetic field pulse in the vicinity of a Feshbach
resonance. This pulse probed the strongly interacting regime for the
condensate, with calculated values for the diluteness parameter (na^3) ranging
from 0.01 to 0.5. The field pulse was observed to cause loss of atoms from the
condensate on remarkably short time scales (>=10 microsec). The dependence of
this loss on magnetic field pulse shape and amplitude was measured. For
triangular pulses shorter than 1 ms, decreasing the pulse length actually
increased the loss, until extremely short time scales (a few tens of
microseconds) were reached. Such time scales and dependencies are very
different from those expected in traditional condensate inelastic loss
processes, suggesting the presence of new microscopic BEC physics.Comment: 4 pages in latex2E, 4 eps figures; revised Fig.1, revised
scatt.lengths, added discussion, new refs., resubmitted to PR
Vector-meson magnetic dipole moment effects in radiative tau decays
We study the possibility that the magnetic dipole moment of light charged
vector mesons could be measured from their effects in \tau^- -->
V^-\nu_{\tau}\gamma decays. We conclude that the energy spectrum and angular
distribution of photons emitted at small angles with respect to vector mesons
is sensitive the effects of the magnetic dipole moment. Model-dependent
contributions and photon radiation off other electromagnetic multipoles are
small in this region. We also compute the effects of the magnetic dipole moment
on the integrated rates and photon energy spectrum of these lepton
decays.Comment: Latex, 12 pages, 4 figures, submitted to PR
Production of cold molecules via magnetically tunable Feshbach resonances
Magnetically tunable Feshbach resonances were employed to associate cold
diatomic molecules in a series of experiments involving both atomic Bose as
well as two spin component Fermi gases. This review illustrates theoretical
concepts of both the particular nature of the highly excited Feshbach molecules
produced and the techniques for their association from unbound atom pairs.
Coupled channels theory provides the rigorous formulation of the microscopic
physics of Feshbach resonances in cold gases. Concepts of dressed versus bare
energy states, universal properties of Feshbach molecules, as well as the
classification in terms of entrance- and closed-channel dominated resonances
are introduced on the basis of practical two-channel approaches. Their
significance is illustrated for several experimental observations, such as
binding energies and lifetimes with respect to collisional relaxation.
Molecular association and dissociation are discussed in the context of
techniques involving linear magnetic field sweeps in cold Bose and Fermi gases
as well as pulse sequences leading to Ramsey-type interference fringes. Their
descriptions in terms of Landau-Zener, two-level mean field as well as beyond
mean field approaches are reviewed in detail, including the associated ranges
of validity.Comment: 50 pages, 26 figures, to be published in Reviews of Modern Physics,
final version with updated reference
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